Gutting: that's one of the words I was looking out for.
Being individually hand made, they are quite collectible. Their nefarious purpose only adds to the value.
They were used by 19th century New England fishermen on the mackerel schooners and are known as mackerel plows. After splitting a scrawny mackerel down its back with a larger knife, a fisherman would make several slashes parallel to the backbone with the small blade of the plow. Like plowing furrows on land, the slashes opened the flesh, causing the fish to swell and look fat, which could bring a higher price. They were also known as fatting knives.